RT8485 - Richtek

®
RT8485
High Voltage High Current LED Driver Controller for
Buck, Boost or Buck-Boost Topology
General Description
Features
The RT8485 is a current-mode PWM controller designed
to drive an external MOSFET for high current LED
applications. With a current sense amplifier threshold of
315mV, the LED current is adjustable with one external
current sense resistor. With the maximum operating input
voltage of 36V and output voltage up to 150V, the RT8485
is ideal for Buck, Boost or Buck-Boost operation.

With the switching frequency programmable over 100kHz
to 1MHz, the external inductor and capacitors can be small
while maintaining high efficiency.
Dimming can be done by either analog or digital. The builtin clamping comparator and filter allow easy low noise
analog dimming conversion from digital signal with only
one external capacitor.
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High Voltage Capability : VIN Up to 36V, LED Sensing
Threshold Common Mode Voltage Up to 150V
Buck, Boost or Buck-Boost Operation
Adjustable Switching Frequency
Easy Dimming Control : Analog or Digital
Converting to Analog with One External Capacitor
Adjustable Soft-Start to Avoid Inrush Current
Adjustable Over-Voltage Protection
VIN Under-Voltage Lockout and Thermal Shutdown
RoHS Compliant and Halogen Free
Ordering Information
RT8485
Package Type
S : SOP-14
Lead Plating System
G : Green (Halogen Free and Pb Free)
The RT8485 is available in the SOP-14 package.
Note :
Applications

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Richtek products are :
General Industrial High Power LED Lighting
Desk Lights and Room Lighting
Building and Street Lighting
Industrial Display Backlight

RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.

Suitable for use in SnPb or Pb-free soldering processes.
Simplified Application Circuit
L1
D1
VIN2
VOUT
CIN
RT8485
VCC
VIN1
Analog
Dimming
SYNC
ISW
ACTL
ISN
VC
CVC
SS
GBIAS
CSS
OVP
RSET
December 2014
LEDs
RSW
RSENSE
R1
RRSET
VOUT
R2
GND
CB
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DS8485-04
COUT
ISP
DCTL
RVC
M1
GATE
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RT8485
Marking Information
Pin Configurations
(TOP VIEW)
RT8485GS : Product Number
RT8485
GSYMDNN
YMDNN : Date Code
RSET
ISW
ISP
ISN
VC
ACTL
DCTL
14
2
13
3
12
4
11
5
10
6
9
7
8
GATE
GBIAS
GND
VCC
OVP
SYNC
SS
SOP-14
Functional Pin Description
Pin No. Pin Name
Pin Function
1
RSET
Switching Frequency Setting. Connect a resistor from RSET to GND. RRSET = 30k will set
f SW = 360kHz.
2
ISW
Current Sense for External MOSFET Switch. Connect the current sense resistor between
external N-MOSFET switch and the ground.
3
ISP
LED Current Sense Amplifier Positive Input with Common Mode Up to 150V.
4
ISN
LED Current Sense Amplifier Negative Input. Voltage threshold between ISP and ISN is
315mV with common mode voltage up to 150V.
5
VC
Compensation Node for PWM Control Loop.
6
ACTL
Analog Dimming Control. The effective programming voltage range of the pin is between
0.2V and 1.2V.
7
DCTL
Digital Dimming Control by adding a 0.47F filtering capacitor on ACTL pin, the PWM
dimming signal on DCTL pin can be averaged and converted into analog dimming signal on
the ACTL pin.
8
SS
Soft-Start Time Setting. A capacitor of at least 10nF is required for proper soft-start.
9
SYNC
Switching Frequency Synchronization Pin. In order to synchronize RT8485 switching
frequency to external frequency, the SYNC pin must be fed with square wave with frequency
higher than the set switching frequency of RT8485. The high level voltage of the square
wave must be higher than1.4V. The SW pin will be pulled low (turned off) on the rising edge
(from low to high) of the SYNC pin signal to achieve frequency synchronization. Keep the
SYNC pin floating if the switching frequency synchronization function is not used.
10
OVP
Over-Voltage Protection Sense Input. The PWM converter turns off when the voltage of the
pin goes higher than 1.18V.
11
VCC
Power Supply of the Chip. For good bypass, a low ESR capacitor is required.
12
GND
Ground. The Exposed Pad must be Soldered to a Large PCB and Connected to GND for
Maximum Power Dissipation.
13
GBIAS
Internal Gate Driver Bias. A good bypass capacitor is required.
14
GATE
External MOSFET Switch Gate Driver Output.
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DS8485-04
December 2014
RT8485
Function Block Diagram
VCC
8.5V
SYNC
RSET
VCC
+
-
+
1.4V
GBIAS
-
OSC
S
4.5V
GATE
Q
+
R
OVP
1.18V
+
R
-
R
+
-
-
VC
270mV
+
ISW
ISN
ISP
GM
+
6µA
SS
DCTL
1.2V
+
+
-
-
GND
ACTL
VISP – VISN
(mV)
315
0
0.2
1.2
VACTL (V)
Figure 4
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December 2014
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RT8485
Operation
The start up voltage of the RT8485 is around 4.5V. When
VCC voltage is greater than 4.5V, the RT8485 starts
operation and a regulated GBIAS supply voltage is
generated by an internal LDO circuit. With VCC greater
than 10V, the GBIAS supply will be regulated around 8.5V
to supply the power for the internal GATE pin driver circuit.
The RT8485 is a constant switching frequency PWM
controller. The OSC block generates an adjustable
switching frequency set by an external resistor at RSET
pin. The RT8485 is also equipped with switching frequency
synchronization function. The switching frequency can be
synchronized to the frequency of the signal feeding into
the SYNC pin. On the rising edge of the SYNC pin signal
toggled from low to high (the high level voltage must be
higher than 1.4V), the switch will be turned off. In order to
make the switching frequency synchronization function
to work, the frequency of the signal feeding into the SYNC
pin must be greater than the nominal switching frequency
set by the resistor at the RSET pin.
The RT8485 features high voltage LED driver control. The
common mode operation voltage of the ISP and ISN pins
can be high up to 150V. The regulated (VISP − VISN) sense
threshold voltage is around 315mV. If the sensed (VISP −
VISN) voltage is lower than 315mV, the VC pin will be
charged higher by the internal OP AMP in the PWM
control loop and vice versa. By the PWM closed loop
control, the (VISP − VISN) voltage is regulated to 315mV.
The actual LED output current can be adjusted by the
sense resistor between the ISP and ISN pins.
The dimming can be done by varying the ACTL/DCTL pin
voltage signal. The internal sense threshold reference for
(VISP − VISN) regulation follows the ACTL/DCTL signal to
achieve dimming control.
The fault protection features of the RT8485 include (1)
VCC Under-Voltage Lockout (UVLO) (2) VOUT Over Voltage Protection (OVP) (3) switch Over-Current
Protection (OCP) (4) Over-Temperature Protection (OTP).
As the system starts, the capacitor at the soft-start pin is
slowly charged by an internal current source around 6μA.
During soft-start period, the VC pin voltage follows the
soft-start pin voltage up by one VBE and gradually ramps
up. The slowly rising VC pin voltage allows the PWM duty
to increase gradually to achieve soft-start function.
In normal operation, the GATE turns high when the
oscillator (OSC) turns high. The ISW pin voltage is the
triangular feedback signal of the sensed switch current
(which equals inductor current ramp). The PWM
comparator compares the ISW pin voltage to the VC pin
voltage. When the ISW pin voltage exceeds the VC pin
voltage, the PWM comparator resets the latch and turns
off GATE. If the ISW pin voltage does not exceed the VC
pin voltage by the end of the switching cycle, the GATE
will be turned off by the OSC circuit for a minimum off
time. The cycle repeats when the GATE is turned on at
the beginning of the next OSC cycle.
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is a registered trademark of Richtek Technology Corporation.
DS8485-04
December 2014
RT8485
Absolute Maximum Ratings
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(Note 1)
Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------GBIAS, GATE -------------------------------------------------------------------------------------------------------------ISW --------------------------------------------------------------------------------------------------------------------------ISP, ISN ---------------------------------------------------------------------------------------------------------------------DCTL, ACTL, OVP (Note 2) ------------------------------------------------------------------------------------------SYNC -----------------------------------------------------------------------------------------------------------------------SS, RSET, VC -------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
SOP-14 ---------------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 3)
SOP-14, θJA ----------------------------------------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 4)
HBM (Human Body Model), except the HV pins ISP (500V) and ISN (500V) ------------------------------MM (Machine Model) -----------------------------------------------------------------------------------------------------
Recommended Operating Conditions



−0.3V to 38V
−0.3V to 10V
−0.3V to 1V
−0.3V to 180V
−0.3V to 8V
−0.3V to 20V
−0.3V to 5V
0.87W
113.9°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 5)
ISP, ISN ---------------------------------------------------------------------------------------------------------------------- 150V
Supply Input Voltage Range, VCC ------------------------------------------------------------------------------------- 4.5V to 36V
Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C
Electrical Characteristics
(VCC = 24V, No Load on any Output, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
--
6
7.2
mA
Overall
Supply Current
SYNC Input
Voltage
IVCC
VVC  0.4V (Switching Off)
Logic-High
VSYNCH
2
--
--
Logic-Low
VSYNCL
--
--
0.8
--
--
1.2
A
302
315
328
mV
SYNC Input Current
SYNC > 2V
V
Current Sense Amplifier
VACTL = 1.4V,
12V  Common Mode  150V
Input Threshold (VISP  VISN)
ISP Input Current
IISP
4.5V  VISP  150V
--
140
--
ISN Input Current
IISN
4.5V  VISN  150V
--
60
--
VC Output Current
IVC
VISP  VISN = 315mV,
0.5V  VC  2.4V
--
20
--
A
--
0.7
--
V
VC Threshold for PWM
Switch Off
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December 2014
A
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RT8485
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
VACTL = 1.2V
--
1
--
VACTL = 0.2V
--
10
--
VACTL_On
--
1.2
1.5
V
VACTL_Off
0.15
0.2
--
V
A
LED Dimming
Analog Dimming ACTL Pin
Input Current
Maximum LED Current On
Threshold at ACTL
LED Current Off Threshold at
ACTL
DCTL Input Current
DCTL Input Voltage
IACTL
A
IDCTL
0.3V  VDCTL  6V
--
--
0.5
VDCTL_H
(Note 6)
2
--
--
VDCTL_L
(Note 6)
--
--
0.3
f SW
RRSET = 30k
280
360
420
kHz
RRSET = 30k
--
250
--
ns
IGBIAS = 20mA
7.8
8.5
9.2
V
IGATE = 50mA
--
7.2
--
IGATE = 100A
--
7.8
--
IGATE = 50mA
--
0.25
--
IGATE= 100A
--
0.1
--
1nF Load at GATE
2
15
300
ns
235
270
305
mV
--
1.18
--
V
V
PWM Control
Switching Frequency
Minimum Off-Time
Switch Gate Driver
GBIAS Voltage
VGBIAS
GATE Voltage High
VGATE_H
GATE Voltage Low
VGATE_L
GATE Drive Rise and Fall Time
PWM Switch Current Limit
ISW_LIM
Threshold
OVP and Soft-Start
VC = 1V
V
V
OVP Threshold
VOVP_th
OVP Input Current
IOVP
0.9  VOVP  1.5V
--
--
0.5
A
Soft-Start Current
ISS
VSS  2V
--
6
--
A
Thermal Shutdown Threshold
TSD
--
145
--
Thermal Shutdown Hysteresis
TSD
--
10
--
C
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the
operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may affect
device reliability.
Note 2. If connected with a 20kΩ serial resistor, ACTL and DCTL can go up to 36V.
Note 3. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is
measured at the exposed pad of the package.
Note 4. Devices are ESD sensitive. Handling precaution is recommended.
Note 5. The device is not guaranteed to function outside its operating conditions.
Note 6. Guaranteed by design, not subjected to production test.
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RT8485
Typical Application Circuit
L1
22µH
VIN
4.5V to 36V
CIN
10µF
RT8485
11
9 SYNC
Analog
Dimming
7 DCTL
5
VC
8
SS
13 GBIAS
RVC
10k
CVC
3.3nF
CSS
0.1µF
CB
1µF
RSW
0.05
ISW 2
3
ISP
4
ISN
10
OVP
6 ACTL
GND
12
RSET
1
VOUT
150V (Max.)
COUT
1µF
M1
GATE 14
VCC
RSENSE
0.47
D1
LEDs
R1
RRSET
30k
VOUT
R2
Figure 1. Analog Dimming in Boost Configuration
D1
COUT
1µF
VIN2
150V (Max.)
VIN1
4.5V to 36V C
IN1
10µF
RSENSE
0.47
RT8485
11
ISP
VCC
Analog
Dimming
6 ACTL
CSS
0.1µF
8
SS
13 GBIAS
CB
1µF
L1
22µH
3
ISW 2
7 DCTL
5 VC
RVC
10k
……
LEDs
ISN 4
GATE 14
9 SYNC
CVC
3.3nF
CIN2
RSET 1
M1
RSW
0.05
RRSET
30k
OVP 10
GND
12
Figure 2. Analog Dimming in Buck Configuration
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RT8485
VIN2
90V to 150V
L1
22µH
CIN
10µF
VIN1
4.5V to 36V
Analog
Dimming
RVC
10k
CVC
3.3nF
CSS
0.1µF
RT8485
M1
11 VCC
GATE 14
9 SYNC
ISW 2
4
6 ACTL
ISN
3
7 DCTL
ISP
10
OVP
5
VC
1 RRSET
RSET
8
SS
13 GBIAS
GND 12
D1
VOUT
150V (Max.)
COUT
1µF
RSW
0.05
LEDs
RSENSE
0.47
R1
VOUT
R2
CB
1µF
Figure 3. Analog Dimming in Buck-Boost Configuration
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RT8485
Typical Operating Characteristics
Efficiency vs. Output Voltage
Efficiency vs. Input Voltage
96
Boost
94
94
92
92
Efficiency (%)
Efficiency (%)
96
90
88
86
84
Buck
90
88
86
84
VIN = 30V, VOUT = 48V to 150V,
IOUT = 150mA, L = 68μH
82
80
VIN2 = 130V to 265V, VOUT = 120V,
IOUT = 150mA, L = 47μH
82
80
45
60
75
90
105
120
135
150
120
140
160
Output Voltage (V)
89
240
260
280
36
40
Supply Current vs. VCC
Buck-Boost
4.0
Supply Current (mA)
Efficiency (%)
220
4.2
88
87
86
85
84
83
82
3.8
3.6
3.4
3.2
VIN = 12V to 30V, VOUT = 20V,
IOUT = 400mA, L = 47μH
81
80
3.0
12
14
16
18
20
22
24
26
28
4
30
8
12
16
Input Voltage (V)
20
24
28
32
VCC (V)
Supply Current vs. Temperature
Switching Frequency vs. VCC
7
Switching Frequency (kHz)1
368.0
6
Supply Current (mA)
200
Input Voltage (V)
Efficiency vs. Input Voltage
90
180
5
4
3
2
367.5
367.0
366.5
366.0
365.5
365.0
364.5
RSET = 30kΩ
1
364.0
-40
-20
0
20
40
60
80
100
120
Temperature (°C)
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DS8485-04
December 2014
140
4
8
12
16
20
24
28
32
36
40
VCC (V)
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RT8485
VISP - VISN Threshold vs. VCC
320
390
318
VISP - VISN Threshold (mV)
Switching Frequency (kHz)
Switching Frequency vs. Temperature
400
380
370
360
350
340
330
320
310
314
312
310
308
306
304
302
RSET = 30kΩ
300
316
300
-40
-20
0
20
40
60
80
100
120
140
4
8
12
16
Temperature (°C)
VISP - VISN Threshold vs. Temperature
24
28
32
36
40
LED Current vs. ACTL Voltage
320
160
318
140
316
LED Current (mA)
VISP - VISN Threshold (mV)
20
VCC (V)
314
312
310
308
306
304
100
80
60
40
302
20
300
0
-50
-25
0
25
50
75
100
125
VOUT = 120V
120
VIN = 30V, IOUT = 150mA
0.2
0.4
0.6
0.8
1
1.2
1.4
Temperature (°C)
ACTL Voltage (V)
LED Current vs. DCTL PWM Duty
DCTL Dimming on PWM Duty 50%
1.6
160
LED Current (mA)
140
120
IOUT
(100mA/Div)
VOUT = 120V
100
IL
(2A/Div)
80
60
40
20
VIN = 30V, IOUT = 150mA
0
0
10
20
30
40
50
60
70
80
90
100
PWM
(5V/Div)
IOUT = 150mA, PWM Duty = 50%, L = 68μH
Time (25μs/Div)
DCTL PWM Duty (%)
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RT8485
DCTL Dimming on PWM Duty 80%
IOUT
(100mA/Div)
Power On form VCC Voltage
V CC
(10V/Div)
IL
(2A/Div)
GATE
(10V/Div)
PWM
(5V/Div)
IOUT = 150mA, PWM Duty = 80%, L = 68μH
Time (5μs/Div)
IOUT
(200mA/Div)
IOUT = 150mA, PWM Duty = 50%, L = 68μH
Time (10ms/Div)
Power Off form VCC Voltage
V CC
(10V/Div)
GATE
(10V/Div)
IOUT
(200mA/Div)
IOUT = 150mA, PWM Duty = 50%, L = 68μH
Time (10ms/Div)
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RT8485
Applications Information
The RT8485 is a current mode PWM controller designed
to drive an external MOSFET for high current LED
applications. The LED current can be programmed by an
external resistor. The input voltage range of the RT8485
can be up to 36V and the output voltage can be up to
150V. The RT8485 provides analog and PWM dimming to
achieve LED current control.
GBIAS Regulator and Bypass Capacitor
The GBIAS pin requires a capacitor for stable operation
and to store the charge for the large GATE switching
currents. Choose a 25V rated low ESR, X7R or X5R
ceramic capacitor for best performance. The value of a
1μF capacitor will be adequate for many applications.
Place the capacitor close to the IC to minimize the trace
length to the GBIAS pin and also to the IC ground. An
internal current limit on the GBIAS output protects the
RT8485 from excessive on-chip power dissipation.
The GBIAS pin has its own under-voltage disable (UVLO)
set to 4.3V(typical) to protect the external FETs from
excessive power dissipation caused by not being fully
enhanced. If the input voltage, VIN, will not exceed 8V,
then the GBIAS pin should be connected to the input
supply. Be aware if GBIAS supply is used to drive extra
circuits besides RT8485, typically the extra GBIAS load
should be limited to less than 10mA.
Loop Compensation
An external resistor in series with a capacitor is connected
from the VC pin to GND to provide a pole and a zero for
proper loop compensation. The external inductor, output
capacitor and the compensation resistor and capacitor
determine the loop stability. The inductor and output
capacitor are chosen based on performance, size and cost.
The compensation resistor and capacitor at VC are
selected to optimize control loop response and stability.
For typical LED applications, a 3.3nF compensation
capacitor at VC is adequate, and a series resistor should
always be used to increase the slew rate on the VC pin to
maintain good regulation of LED current during fast
transients on the input supply to the converter The typical
compensation for the RT8485 is 10kΩ and 3.3nF.
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Soft-Start
The soft-start of the RT8485 can be achieved by connecting
a capacitor from SS pin to GND. The built-in soft-start
circuit reduces the start-up current spike and output
voltage overshoot. The soft-start time is determined by
the external capacitor charged by an internal 6μA constant
charging current. The SS pin directly limits the rate of
voltage rise on the VC pin, which in turn limits the peak
switch current.
The soft-start interval is set by the soft-start capacitor
selection according to the equation :
2.4V
tSS  CSS 
6μA
A typical value for the soft-start capacitor is 0.1μF. The
soft-start capacitor is discharged when VCC falls below
its UVLO threshold, during an over temperature event or
during an GBIAS under voltage event.
LED Current Setting
The LED current is adjustable by placing an appropriate
value current sense resistor between the ISP and ISN pins.
Typically, sensing of the current should be done at the
top of the LED string. The ACTL pin should be tied to a
voltage higher than 1.2V to get the full-scale 315mV
(typical) threshold across the sense resistor. The ACTL
pin can also be used to dim the LED current to zero,
although relative accuracy decreases with the decreasing
voltage sense threshold. When the ACTL pin voltage is
less than 1.2V, the LED current is :
ILED 
(VACTL  0.2)  0.315
RSENSE
Where,
RSENSE is the resistor between ISP and ISN.
When the voltage of ACTL is higher than 1.2V, the LED
current is regulated to :
ILED(MAX) 
315mV
RSENSE
The ACTL pin can also be used in conjunction with a
thermistor to provide over-temperature protection for the
LED load, or with a voltage divider to VIN to reduce output
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December 2014
RT8485
power and switching current when VIN is low. The presence
of a time varying differential voltage signal (ripple) across
ISP and ISN at the switching frequency is expected.
800
700
600
500
400
300
200
Switching Frequency Setting
The RSET frequency adjust pin allows the user to adjust
the switching frequency from 100kHz to 1MHz for optimized
efficiency and performance or external component size.
Higher frequency operation allows smaller component size
but increases switching losses and gate driving current,
and may not allow sufficient high or low duty cycle
operation. Lower frequency operation gives better
performance but with larger external component size. For
an appropriate RRSET resistor value see Table 1 or Figure
4. An external resistor from the RSET pin to GND is
required and do not leave this pin open.
100
0
0
15
30
45
60
75
90
105
120
135
R
RRSET
(kΩ)
RSET (kΩ)
Figure 4. Switching Frequency vs. RRSET
Output Over Voltage Setting
fOSC (kHz)
RRSET (k)
1000
8
800
10
The RT8485 is equipped with Over-Voltage Protection
(OVP) function. When the voltage at OVP pin exceeds a
threshold of approximately1.18V, the power switch is
turned off. The power switch can be turned on again once
the voltage at OVP pin drops below 1.18V. For the Boost
and Buck-Boost application, the output voltage could be
clamped at a certain voltage level. The OVP voltage can
be set by the following equation :
R1 

VOUT, OVP  1.18   1 

 R2 
600
15
Where,
500
19
300
35
R1 and R2 are the voltage dividers from VOUT to GND with
the divider center node connected to OVP pin.
200
55
100
120
Table 1. Switching Frequency vs. RREST Value (1%
Resistor)
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DS8485-04
900
Frequency (kHz)1
The amplitude of this signal is increased by high LED load
current, low switching frequency and/or a smaller value
output filter capacitor. The compensation capacitor on the
VC pin filters the signal so the average difference between
ISP and ISN is regulated on the user-programmed value.
Frequency vs. RRSET
1000
December 2014
Over-Temperature Protection
The RT8485 has Over-Temperature Protection (OTP)
function to prevent the excessive power dissipation from
overheating. The OTP function will shut down switching
operation when the die junction temperature exceeds
145°C. The chip will automatically start to switch again
when the die junction temperature cools off.
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RT8485
Inductor Selection
The converter operates in discontinuous conduction mode
when the inductance value is less than the value LBCM.
With an inductance greater than LBCM, the converter
operates in Continuous Conduction Mode (CCM). The
inductance LBCM is determined by the following equations.
For Buck application :
VOUT
 VIN  VOUT 
LBCM 


2  IOUT  f 
VIN

For Boost application :
 VOUT  VIN 
VIN2
LBCM 
 

2  IOUT  f  VOUT 2 
For Buck-Boost application :
VIN2
VOUT
LBCM 

2  IOUT  f  VIN  VOUT 2
where
VOUT = output voltage.
VIN = input voltage.
f = operating frequency.
IOUT = LED current.
Choose an inductance based on the operating frequency,
input voltage and output voltage to provide a current mode
ramp signal during the MOSFET on period for PWM control
loop regulation. The inductance also determines the
inductor ripple current. Operating the converter in CCM is
recommended, which will have the smaller inductor ripple
current and hence the less conduction losses from all
converter components.
As a design example, to design the peak to peak inductor
ripple to be ±30% of the output current, the following
equations can be used to estimate the size of the needed
inductance :
For Buck application :
VOUT
 VIN  VOUT 
L=


2  0.3  IOUT  f 
VIN

For Boost application :
 VOUT  VIN 
VIN2
L=
 

2  0.3  IOUT  f  VOUT 2 
For Buck-Boost application :
VIN2
VOUT
L=

2  0.3  IOUT  f  VIN  VOUT 2
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14
The inductor must also be selected with a saturation
current rating greater than the maximum inductor current
during normal operation. The maximum inductor current
can be calculated by the following equations.
For Buck application :
VOUT  VIN  VOUT 
IPEAK = IOUT +


2  L  f 
VIN

For Boost application :
VOUT  IOUT
VIN
 VOUT  VIN 
IPEAK =
+


2  L  f  VOUT
  VIN

For Buck-Boost application :
IPEAK =
 VIN  VOUT   IOUT
  VIN
+
VIN
 VOUT


2  L  f  VIN  VOUT 
where
η is the efficiency of the power converter.
Power MOSFET Selection
For applications operating at high input or output voltages,
the power N-MOSFET switch is typically chosen for drain
voltage VDS rating and low gate charge. Consideration of
switch on-resistance, R DS(ON), is usually secondary
because switching losses dominate power loss. The
GBIAS regulator on the RT8485 has a fixed current limit
to protect the IC from excessive power dissipation at high
VIN, so the N-MOSFET should be chosen so that the
product of Qg at 5V and switching frequency does not
exceed the GBIAS current limit.
ISW Sense Resistor Selection
The resistor, RSW, between the Source of the external NMOSFET and GND should be selected to provide adequate
switch current to drive the application without exceeding
the current limit threshold set by the ISW pin sense
threshold of RT8485. The ISW sense resistor value can
be calculated according to the formula below :
Current Limlit Threshold Minimum Value
IOCP
where IOCP is about 1.33 to 1.5 times of inductor peak
current IPEAK.
RSW 
The placement of RSW should be close to the source of
the N-MOSFET and the IC GND of the RT8485. The ISW
pin input to RT8485 should be a Kelvin sense connection
to the positive terminal of RSW.
is a registered trademark of Richtek Technology Corporation.
DS8485-04
December 2014
RT8485
The Schottky diode, with their low forward voltage drop
and fast switching speed, is necessary for the RT8485
applications. In addition, power dissipation, reverse voltage
rating and pulsating peak current are the important
parameters for the Schottky diode selection. Choose a
suitable Schottky diode whose reverse voltage rating is
greater than maximum output voltage. The diode's average
current rating must exceed the average output current.
The diode conducts current only when the power switch
is turned off (typically less than 50% duty cycle). If using
the PWM feature for dimming, it is important to consider
diode leakage, which increases with the temperature, from
the output during the PWM low interval. Therefore, choose
the Schottky diode with sufficiently low leakage current.
Capacitor Selection
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most of the RT8485 applications, a 10μF ceramic capacitor
is sufficient. A value higher or lower may be used
depending on the noise level from the input supply and
the input current to the converter.
In Boost application, the output capacitor is typically a
ceramic capacitor and is selected based on the output
voltage ripple requirements. The minimum value of the
output capacitor COUT is approximately given by the
following equation :
COUT 
IOUT  VOUT
VIN  VRIPPLE  fSW
For LED applications, the equivalent resistance of the LED
is typically low and the output filter capacitor should be
sized to attenuate the current ripple. Use of X7R type
ceramic capacitors is recommended. Lower operating
frequencies will require proportionately higher capacitor
values.
maximum power dissipation can be calculated by the
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction to ambient
thermal resistance.
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
SOP-14 package, the thermal resistance, θ JA , is
113.9°C/W on a standard JEDEC 51-7 four-layer thermal
test board. The maximum power dissipation at TA = 25°C
can be calculated by the following formula :
PD(MAX) = (125°C − 25°C) / (113.9°C/W) = 0.87W for
SOP-14 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curves in Figure 5 allow the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
1.6
Maximum Power Dissipation (W)1
Schottky Diode Selection
Four-Layer PCB
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 5. Derating Curve of Maximum Power Dissipation
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS8485-04
December 2014
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15
RT8485
Layout Consideration

PCB layout is very important to design power switching
converter circuits. The layout guidelines are suggested
as follows :
The input capacitor CVCC must be placed as close to
VCC pin as possible.

Place the compensation components to VC pin as close
as possible to avoid noise pick up.

Connect GND pin and Exposed Pad to a large ground
plane for maximum power dissipation and noise
reduction.

The power components L1, D1, CIN, M1 and COUT must
be placed as close to each other as possible to reduce
the ac current loop area. The PCB trace between power
components must be as short and wide as possible
due to large current flow through these traces during
operation.
Place these components
as close as possible
D1
Power trace must be wide
and short when compared
to the normal trace.
VIN power trace to L1
must be wide and short.
L1
VIN
M1
COUT
GND
RSW
GND
RSENSE
RSET
ISW
ISP
ISN
VC
ACTL
DCTL
14
2
13
3
12
4
11
5
10
6
9
7
8
GATE
GBIAS
GND
VCC
OVP
SYNC
SS
RVC
CIN
CVCC
The input capacitor as
close VCC pin as possible.
Normal trace.
CSS
CVC
GND
Locate The compensation
components to VC pin as
close as possible.
Figure 6. PCB Layout Guide
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is a registered trademark of Richtek Technology Corporation.
DS8485-04
December 2014
RT8485
Outline Dimension
H
A
M
J
B
F
C
I
D
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
8.534
8.738
0.336
0.344
B
3.810
3.988
0.150
0.157
C
1.346
1.753
0.053
0.069
D
0.330
0.508
0.013
0.020
F
1.194
1.346
0.047
0.053
H
0.178
0.254
0.007
0.010
I
0.102
0.254
0.004
0.010
J
5.791
6.198
0.228
0.244
M
0.406
1.270
0.016
0.050
14–Lead SOP Plastic Package
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot
assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be
accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
DS8485-04
December 2014
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